1. Field of the Invention
The present invention relates to an MCP unit having a multiplying function of charged particles such as electrons and ions, an MCP detector including the MCP unit, and a time-of-flight mass spectrometer including the MCP detector, as relevant parts of a detector used for time-of-flight mass spectrometry or the like.
2. Related Background Art
As a method of detecting a polymer molecular weight, time-of-flight mass spectrometry (TOF-MS) is known. FIG. 1 is a diagram for describing a configuration of an analyzing device (hereinafter, referred to as a TOF-MS device) by the TOF-MS.
As shown in FIG. 1, in the TOF-MS device, a detector 100 is arranged at one end in a vacuum chamber 110, and a sample (ion source) 120 is arranged at the other end in the vacuum chamber 110. Between the detector 100 and the sample 120, a ring-shaped electrode 130 (ion accelerator) having an opening is arranged. The electrode 130 is grounded, and when the sample 120 to which a predetermined voltage is being applied is irradiated with a laser beam from an ion extracting system (that includes a laser light source), ions released from the sample 120 are accelerated by an electric field formed between the sample 120 and the electrode 130 and collide with the detector 100. Acceleration energy applied to the ions between the sample 120 and the electrode 130 is determined by an ionic charge. Thus, when the ionic charge is identical, a velocity achieved when the ionic charge passes through the electrode 130 depends on the weights of ions. Additionally, between the electrode 130 and the detector 100, the ions travel at a constant velocity. Thus, a time of flight of the ions between the electrode 130 and the detector 100 is inversely proportional to the velocity. That is, an analyzing section calculates the time of flight from the electrode 130 to the detector 100 to determine the weights of ions (an output voltage from the detector 100 is monitored with an oscilloscope). Visually, it becomes possible to determine the weights of ions from an occurrence time of a peak appearing in a time spectrum of the output voltage displayed on the oscilloscope.
As a detector applicable to such a TOF-MS device, an MCP detector disclosed in Japanese Patent Application Laid-Open No. H6-28997 (reference document 1), for example, is known. FIG. 2 is a schematic cross-sectional view showing one example of an MCP detector applicable to the TOF-MAS device. In an MCP detector 100a shown in FIG. 2, two micro-channel plates (MCP) 20 and 21 (hereinafter, referred to as an MCP cluster 2) are sandwiched by an IN-electrode 1 and an OUT-electrode 3, each of which is formed with an opening at its center. Before the IN-electrode 1, while a wire mesh-like grid electrode 106 held by a frame 105 is arranged, behind the OUT-electrode 3, an anode electrode 4 is arranged. Further, on a shield side of a signal-reading BNC terminal (Bayonet Neil-Concelman connector) 60, a casing 5x comprised of a conductive material is connected while on a core wire 601 side, an electrode 47 is connected. Between the casing 5x and the OUT-electrode 3, and between the electrode 47 and the anode electrode 4, dielectrics 22 and 46 are arranged, respectively, thereby to form capacitors.
In the MCP detector 100a having the above-described structure, when charged particles are incident upon the MCP cluster 2, a great number of electrons (secondary electrons multiplied by the respective MCPs) are released from the MCP cluster 2 in response thereto. The secondary electrons thus released reach the anode electrode 4 and are then converted into an electric signal as a change of voltage or current (a signal is outputted from the core wire 601). At this time, the capacitor is formed between the anode electrode 4 and the core wire 601. Thus, a detection signal is outputted to the outside by a ground potential, and the existence of the capacitor formed between the casing 5x and the OUT-electrode 3 inhibits occurrence of waveform distortion or ringing of the output signal.